Method of applying copper coatings to uranium



lVIETHOD F APPLYING COPPER COATINGS T O URANIUM 7 Claims. (Cl. 204-15) This invention'relates to the applicationof protective coatings on metallic uranium. The products produced in practicing this invention are useful in a neutronic reactor of the type described in the co-pending application of Fermi et al., S.N. 568,904, filed December 19, 1944; now Patent No. 2,708,656, dated May 17, 1955.

' Metallic uranium'is an easily oxidizable metal which is readily corroded by atmospheric oxygen and by aqueous solutions. In order to protect the metal from the injurious effects of such media, it is desirable to provide a coating of some material which is less easily acted upon.

Many common protective coating materials are unsuitable for coating uranium; lead forms a pyrophoric alloy with uranium and for this reason its use is objectionable; some metals, such as tin, permit diffusion of the uranium through coatings of these metals with consequent impairment of their protective value; other metals form with the uranium, very brittle alloys containing many cracks and crevices which reduce" their efiectiveness.

It is an object of the present invention to provide protective coatings for uranium which are not subject to the defects enumerated above. A further object is to provide a bar rier coating for uranium over which it is possible to apply various protective coatings that normally would be unsuitable because of their behavior toward the base metal. A further object .of the invention is to provide highly protective electrodeposited coatings on metallic uranium.

In accordance with the present invention metallic ma nium is provided with a protective coating comprising copper. The copper may be applied directly to the metal or to an intermediate coating of some other suitable metal possessing the property of adhering firmly to uranium and to copper. Nickel may be used to advantage for this purpose.

The metallic uranium preferably is prepared for the application of the coating by an etchant treatment of the metal surface with an aqueous etching solution containing chloride ions, for example, a hydrochloric acid solution or a molten hydrate of ferric chloride. Particularly satisfactory treatment of this type involve anodic pickling of the metal in aqueous trichloracetic acid so lution, or an aqueous phosphoric acid solution containing about /2% of hydrochloric acid, to remove a /2 to one mil layer of the surface metal. Prior to the etchtroplating from conventional copper electroplating baths,

such as acid copper, cyanide copper or pyrophosphate copper plating baths. The copper coatings of the invention may vary from exceedingly thin coatings of the or-' der of 0.000015 inch up to 0.001 inch or more. The thickness of the coating should be selected appropriately to suit the subsequent treatment of the metal. Thus the thinnest coatings are normally employed in combination with relatively heavy coatings of other coating metals, whereas the heavy coatings are employed alone or in combination with additional metal coatings of moderate thickness.

The copper protective coatings are especially advantageous in that they inhibit diffusion of metallic uranium into other metals separated from the uranium by the copper. Thus the copper is particularly valuable as a. barrier for preventing difiusion of uranium into protective coatings of lead, zinc, tin, cadmium, aluminum, and their alloys, and other metals relatively low resistance to uranium diffusion. j 'In my US. patent application Serial No. 572,093 entitled, Nickel Coatings and Method of Applying, filed January 9, 1945, now Patent No. 2,854,738, date Octoher 7, 1958, I have described uranium products provided with nickel protective coatings. Although the copper coatings of the present invention are superior to ant treatment the metal may be cleaned by sand-blast- .;...The. metallic copper. coatingmay he applied by elecfrom 0.000015 inch to 0.00005 inch, are suitable.

nickel coatings with respect to the barrier eilect which they have upon diffusion of uranium, they can be substantially improved from the standpoint of corrosion resistance by combination with nickel coatings. Highly satisfactory combinations are disclosed and claimed in my application aforesaid. Other useful combinations are described and claimed herein.

Nickel provides coatings which adhere more firmly to uranium than copper coatings and consequently coatings of maximum adherence may be obtained by applying a first coating of nickel and a second coating ofcop per over the nickel. In this case the nickel is employed for its adhesive ability rather than its corrosion resistance.

Corrosion resistance may be secured by means of copper alone or by means of corrosion-resistant coatings ap .plied over the copper. For example, copper constitutes a suitable base for application of lead coatings, which provide protection against a wide variety of corrosive agents. The copper, by barring difiusion of uranium into the lead coatings, eliminates the possibility of the formation of a pyrophoric lead-uranium alloy which might otherwise be produced. The uranium-copper-lead bond is also more secure than the uranium-lead bond, and a uranium-nickel-copper-lead bond is even more satisfactory.

While the protective value of lead alone is consid erable, electroplated lead coatings have a somewhat porous structure permitting diifusion of corrosive agents into the plating and consequently provide a less efiective protection than is obtained with an impervious coating. By the application of a coating of copper and tin or copper and nickel over the lead platings their porosity can be reduced and their corrosion resistance increased.

An especially important improvement can be secured by electroplating a thin coating of tin upon an electroplated copper coating and then heating the tin to a temperature above its melting point but below the melting point of copper to cause the tin to diffuse into the pores of the'copper plating. Very thin tin coatings, commonly termed flash coatings, the thickness of which may be Tin impregnated. copper coatings prepared in this manner have been found to possess corrosion resistance many times greater than simple copper-tin coatings. This technique is applicable to simple copper platings 'upon ura- Patented July 14, 1959' J nium as well as complex coatings such as the series nickel-copper-lead-copper. In the latter case the fusion treatment should be conducted below the melting temperature of the lead plating.

The copper coatings of the present invention have been found to be of very substantial value for use in combination with chromium as a protective coating for uranium. While chromium has been found to produce firmly adherent coatings on smooth metallic uranium surfaces, the throwing power of chromium electroplating solutions is very low; hence, irregularities such as cracks, crevices, pinholes and the like in the metallic uranium surfaces are coated relatively 'slowly and the metallic uranium in such surface irregularities may be appreciably acted upon by the electroplating solution before the electroplated chromium layer attains protective proportions. The copper not only adds to the value of the finished coatings because of its barrier action but also protects fissures in the uranium metal surface from the chromium electroplating bath during the early period of electroplatingwith chromium. In order to afford protection for uranium during the chromium electroplating, his not necessary to provide a thick copper layer since a copper flash coating has been found effective for this purpose.

It is normally expeditious to employ copper alone for the application of copper platings, but it is possible to effect similar results by employing mixed electroplating baths which apply two metals simultaneously. Thus by means of a copper-zinc electroplating bath brass coatings may be applied and these coatings possess barrier properties similar to those possessed by pure copper coatings.

Example 1 A machined uranium rod about 8 inches long and 1% inch in diameter is prepared for electroplating by dipping in trichlorethene to remove grease, sand-blasting lightly, dipping in aqueous 35% HNO by weight at 30 C. for 4 minutes, rinsing with cold water, etching by anodic treatment in aqueous 50%trichloracetic acid solution at between 50 C. and 60 C. for minutes at 50 amperes per square foot current density, rinsing with cold water, dipping for 4 minutes in cold aqueous 35 HNO solution to clean the etched metal surface, then rinsing in cold water.

The cleaned etched metal rod is immediately electroplated in a copper pyrophosphate electroplating bath comprising 75 grams per liter of copper as pyrophosphate at a current density of 25 amperes per square foot at 60 C. with airagitation until a copper plating about 0.001 'inch thick is formed (about 60 minutes).

The uranium, treated as described, has a firmly adherent, continuous coating of copper, which for many purposes provides an adequate degree of protection against corrosion.

Since the copper electroplating is not as impervious as desirable for certain purposes, it is improved inthis respect by electroplating it in an aqueous tin electroplating bathcomprising 90 grams per liter of sodium stannate at a current density of 25 amperes per square foot until a *tinplating about 0.0001 inch thickhas formed (about 10 minutes), then rinsing, drying, and heating at 300 C. for about 3 hours to cause the tin to diffuse into the pores of the copper plating.

Example 2 A uranium rod copper-plated as in Example 1 is given .a 0.001 inch nickel plating in an aqueous nickel sulfate electroplating bath comprising 240 grams per liter of nickel sulfate heptahydrate, 45 grams per liter of nickel chloride hexahydrate, and 30 grams per liter of boric acid, at a current density of 25 amperes per square foot for about 50 minutes. The uranium is thus provided with a protective coating of improved corrosion resistance.

Example 3 An extruded uranium rod about 4 inches longqand 1.1

inch in diameter is prepared for electroplating by dipping in aqueous 36% HQ solution for 20 seconds, then in aqueous 50% I-INO solution for 15 seconds and final-1y in aqueous 36% HCl solution for 2 seconds. It is rinsed in cold water and electroplated in a nickel sulfate electroplating bath of the same composition as described in Example 2 for 15 minutes at about 25 amperes per square foot and ambient temperature to provide a nickel plating about 0.0003 inch thick.

The coated rod is rinsed with Water and electroplated in a pyrophosphate type copper electroplating bath of the composition recited in Example 1 at a current density of 25 amperes per square foot at 60 C. until a flash coating of copper about 0.00003 inch thick has formed.

The copper-plated rod is again rinsed and then electroplated in a lead sulfamate electroplating bath comprising about 250 grams per liter of lead sulfamate at 25 C. at a current density of 25 amperes per square foot until a lead plating about 0.02 inch thick has formed (about 5 hours and 40 minutes).

This coating adheres firmly to the uranium. The copper provides a firm bond between the lead and the nickel and acts as a highly efficient blockinglayer or barrier to diffusion of uranium into the outer lead layer. The thick lead layer provides very good protection against many corrosive agents.

Still greater protection is secured by electroplating a tin coating about 0.00005 inch thick over the lead to reduce its porosity. The tin is applied in the manner described in Example 1. It is preferred to provide an intermediate flash layer of copper byclectroplating the leadcovered rod in an aqueous copper cyanide electroplating bath (a strike) comprising 15 grams of copper cyanide per liter at 40 C. and at 50 amperes per square foot current density for 5 minutes, and to apply the tin over this copper layer. As in the copper-tin coating system of Example l, the tin serves to fill the pores of the copper and this effect is increased and the corrosion resistance of the coating improved by a fusion treatment at about 250 C. for about 3'hours.

A somewhat similar but less effective coating is secured substituting a nickel plating for the tin plating.

A uranium rod is copper-plated as described in Example 1 but for only about 30 minutes. Over the 0.0005 inch copper plating (after rinsing) a chromium plating .is applied by electroplating the copper-plated rod in a chromic acid bath containing 400 grams. per liter :of

chromic acid at 50 C. and 250 amperes per square foot current density until a chromium layer 0.0005 inch thick has formedKabout 30 minutes). A bright, smooth, even,

firmly adherent composite electroplating is thus'formed having especially valuable properties as a protective and barrier coating.

Thisapplication is adivision from-my copending application Serial No. 571,673 filed January 6,'1945,"now

Patent No. 2,854,737, dated October 7, 1958.

It will be understood that Iintended toinclude variations and modifications of the invention,-and that the preceding examples are illustrations only and in nowise to be construed as limitations upon the invention, the

scope of which is defined in the appended claims, wherein I claim:

1. Themethod of protectingimetallic uranium which comprises anodic etching of the uranium in an aqueous phosphoric acid solution containing chloride ions, cleaning the etched uranium in aqueous nitric acid solution, promptly electroplating the cleaned uranium in a copper electroplating bath, and then electroplating thereupon a metal selected from the group consisting of lead, tin, zinc, cadmium, chromium, and nickel from an aqueous electroplating bath.

2. The method of providing an adherent metallic coating on uranium, which comprises anodic etching the uranium by an aqueous phosphoric acid solution containing chloride ions, cleaning the etched uranium in aqueous nitric acid solution, and promptly electroplating the cleaned uranium in -a copper electroplating bath and then in a chromium electroplating bath.

3. The method of providing an adherent metallic coating on uranium, which comprises anodic etching the uranium by an aqueous phosphoric acid solution containing chloride ions, cleaning the etched uranium in aqueous nitric acid solution, and promptly electroplating the cleaned uranium in a pyrophosphate copper electroplating bath and then in a chromic acid electroplating bath.

4. The method of providing an adherent metallic coating on uranium, which comprises etching the uranium by anodic treatment in an aqueous trichloracetic acid solution, cleaning the etched uranium in aqueous nitric acid solution, promptly electroplating the cleaned uranium in a copper pyrophosphate electroplating bath and then in a sodium stannate electroplating bath, and subsequently heating the coated uranium to a temperature above the melting point of tin but below the melting point of copper to fuse the tin plating and cause it to diffuse into the pores of the copper plating.

5. The method of providing an adherent protective coating on a metallic uranium article, which comprises anodic etching the metallic uranium article by an aqueous phosphoric acid solution containing chloride ions, cleaning the etched article in aqueous nitric acid, promptly electroplating the uranium first in a nickel sulfate electroplating bath, then in a pyrophosphate copper electroplating bath, and then in a lead sulfamate electroplating bath.

6. The method of providing an adherent protective coating on a metallic uranium article, which comprises anodic etching the metallic uranium article by an aqeous phosphoric acid solution containing chloride ions, cleaning the etched article in aqueous nitric acid, and promptly electroplating the uranium successively in a nickel sulfate electroplating bath, a pyrophosphate copper electroplating bath, a lead sulfamate electroplating bath, a copper cyanide electroplating bath, and a tin electroplating bath.

7. The process of claim 6 wherein the tin-electro- W plated uranium article is heated at a temperature above the melting point of tin but below the melting point of lead so as to effect difiusion of the tin into the pores of the preceding cooper plating.

References Cited in the file of this patent UNITED STATES PATENTS 673,126 Martin Apr. 30, 1901 1,147,718 Hall July 27, 1915 1,975,818 Work Oct. 9, 1934 2,128,550 Ford Aug. 30, 1938 2,162,789 Raub June 20, 1939 2,195,499 Schofield Apr. 2, 1940 2,250,556 Stareck 'July 9, 1941 2,555,372 Ramage June 5, 1951 OTHER REFERENCES Transactions of the Electrochemical Society, vol. 66

(*1934), page 41. 

1. THE METHOD OF PROTECTING METALLIC URANIUM, WHICH COMPRISES ANODIC ETCHING OF THE URANIUM IN AN AQUEOUS PHOSPHORIC ACID SOLUTION CONTAINING CHLORIDE IONS, CLEANING THE ETCHED URANIUM IN AQUEOUS NITRIC ACID SOLUTION, PROMPTLY ELECTROPLATING THE CLEANED URANIUM IN A COPPER ELECTROPLATING BATH, AND THEN ELECTROPLATING THEREUPON A METAL SELECTED FROM THE GROUP CONSISTING OF LEAD, TIN, ZINC, CADMIUM, CHROMIUM, AND NICKEL FROM AN AQUEOUS ELECTROPLATING BATH. 